The present invention relates to a process for manufacturing glass objects comprising in particular a step for feeding at least one mould with a glass gob in the viscous liquid state, a step for forming the object in the mould, a step for extracting and conveying the glass object during which the glass object is still at a temperature which is sufficiently high to be deformable, followed by a step for placing the glass object on a transfer belt.
Machines for manufacturing glass objects such as bottles are fed with a molten glass parison whose viscosity is such that this parison may assume its final shape within a given period of time depending on the manufacturing cycle. The glass parison arrives in the mould at a temperature higher than the operating temperature and then is shaped within the mould either mechanically or by blowing, and then the produced glass article is extracted from the mould by suitable means such as tongs in the case of bottles which take hold of the bottle under the lower ledge of the neck which cools more rapidly than the rest of the bottle. The glass object is then conveyed (by means of the tongs) to a standby plate above which it is immobilized. This plate is perforated, and cooling air at ambient temperature is blown in order to accelerate the cooling. When its temperature is lower than the glass deforming temperature, the object is placed on the standby plate, and it is then placed on a transfer belt which conveys the articles into an annealing furnace or any other intermediate machine.
During its forming, the glass object is put in contact with one or more moulds for the purpose of being simultaneously shaped and cooled. Problems arise in respect of the shaped object when the glass, upon contact with the mould, is excessively cooled, when it is cooled too quickly or excessively unevenly or when it is not sufficiently cooled. It is usually found that an excessive or uneven cooling results in corrugations on the skin of the glass articles, while an excessively rapid cooling results in glazing, i.e. the beginning of breaking.
An insufficient cooling causes the glass to adhere to the walls of the mould, and deformation of the glass articles is produced.
The applicants have found that these various problems were due to a bad adjustment of the principal parameters involved in the cooling of the glass, i.e. the temperature of the mould or moulds, the time of contact between the glass and the mould, and/or the pressure of contact between the glass and the mould.
French patent application No. 86/17759 filed on Dec. 18, 1986 and entitled, "Process of manufacturing glass objects" solves the problems of the cooling of the moulds and the glass-mould contact time.
The temperature of the moulds varies cyclically in the course of manufacture. The amplitude of these variations diminishes as one moves away from the surface in contact with the glass. It has been found that this temperature variation is substantially zero on the outer surface of the moulds.
However, it frequently occurs that a certain number of disturbances are added in the course of time to these cyclic variations. The applicants have found that these variations were due to variations in the exterior temperature of the air, unequal cooling from one mould to another, and/or changes in the pressure of the cooling air.
Consequently, it appears that the control of the temperature of the glass and consequently the temperature of the moulds and the cooling of the latter constitute the essential elements if defects on the glass articles in the course of their forming are to be avoided.
It is known from the article entitled "Automatic Control of Mold Cooling Wind" published in the report of the "36th Annual Conference on Glass Problems", to put the pressure of the cooling air under the control the variations in the temperature of this air. Thus, when the temperature of the ambient air increases during the day, for example, the pressure of the cooling air and therefore the flow of the latter is increased while, when the ambient air drops in temperature, for example in the wintertime or during the night, the pressure of the cooling air and therefore the flow of the latter is reduced. According to the results indicated in this article, such a process permits improving the cooling of the moulds on average in the course of time without however solving the problem posed by the various aforementioned disturbances. In the abovementioned technique, the cooling air is generally produced by means of a powerful ventilation which imposes a circulation of air at a high rate of flow around the manufacturing machine.
After having solved the problems discussed hereinbefore, the applicants have found the existence of problems related to the step for extracting and conveying the glass object.
When he attempts to increase the productivity of the machine, in the course of this step, the man skilled in the art will turn to the technique taught in the article referred to hereinbefore, which consists in increasing the pressure of the ventilating air in an attempt to lower the temperature of the glass article more rapidly. This operation causes an increase in the impulse of the cooling fluid which, when it encounters the still-hot object, has a tendency to deform it. This phenomenon is particularly noticeable in respect of hollow bodies having long necks such as bottles. The glass object leaves the mould with a still-high skin temperature. Although the skin would have solidified, the core of the object is still in a deformable state.
Furthermore, the ventilated air ducts and the fan are often of insufficient dimensions which does not permit optimizing the rate of cooling of the glass articles.